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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105841] [12/13 Regression] Change in behavior of CTAD for
 alias templates
Date: Mon, 06 Jun 2022 20:12:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105841

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
                 CC|                            |mike at spertus dot com
           Assignee|jason at gcc dot gnu.org           |unassigned at gcc d=
ot gnu.org

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
In alias_ctad_tweaks, I wrote:

  /* This implementation differs from the [standard] in two significant way=
s:=20=20=20

     1) We include all template parameters of A, not just some.=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20
     2) The added constraint is same_type instead of deducible.=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20

     I believe that while it's probably possible to construct a testcase th=
at=20=20=20
     behaves differently with this simplification, it should have the same=
=20=20=20=20=20=20
     effect for real uses.  Including all template parameters means that we=
=20=20=20=20=20
     deduce all parameters of A when resolving the call, so when we're in t=
he=20=20=20
     constraint we don't need to deduce them again, we can just check wheth=
er=20=20=20
     the deduction produced the desired result.  */

This testcase is an example of why my assumption above was wrong.  The effe=
ct
of the transformations specified by the standard would be roughly

template <class> struct B_deducible;
template <class BT, int BN> struct B_deducible<B<BT,BN>> { };
template <class BT, class... fTs>
auto fp(BT, fTs...) -> A<BT, sizeof...(fTs)>
  requires requires { B_deducible<A<BT,sizeof...(fTs)>>(); };

decltype(fp(0,0)) b(0,0);

which works.

A year ago, Mike Spertus had a partial fix for this; any update on that?=